Accounting for Derivative Instruments and Hedging Activities Derivative Activity and Notional Amounts (Details) MWh in Millions, MMBTU in Millions	3 Months Ended
	Sep. 30, 2018 MWh MMBTU	Dec. 31, 2017 MWh MMBTU
Natural Gas Derivatives [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative, Notional amount, Nonmonetary	302.0	332.1
Electric generation fuel [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative, Notional amount, Nonmonetary	134.6	166.8
Purchased Electricity [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Derivative, Notional amount, Nonmonetary | MWh	6.5	2.9
Subsidiaries [Member]
Derivative [Line Items]
Hedging strategy number of years extended	3 years